Antenna system for a cellular telephone network

ABSTRACT

A cellular telephone system is described of the type wherein a plurality of contiguous cells, each having an assigned set of identification codes, are arranged with means for maintaining continuous communication with mobile telephones moving from cell to cell. The system allows multiple access by including means for assigning at least one of the in the assigned set of identification codes to more than one mobile telephone. A unique identification code is assigned to a mobile telephone located in the cell. A signal having a unique identification code is generated for identifying the mobile telephone. The signal is coupled to the zones. A combiner is also provided for combining signals from all of the zones in the cell. A receiver is coupled to the combiner for retrieving the signals having the code. According to another aspect of the invention, the signal coupled to the zones is delayed so that the transmission of the signal among the plurality of antenna sets is delayed by a preselected amount so as to reduce interference caused by successive reception of signals.

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 07/679,521.

FIELD OF THE INVENTION

[0002] This invention relates generally to cellular telephone systems.More particularly, the invention relates to a digital multiple accesscommunication system for cellular telephone systems.

BACKGROUND OF THE INVENTION

[0003] In a typical analog cellular telephone system, a plurality ofcontiguous cells, each having a different assigned set of transmissionfrequencies, are arranged with handoff means for maintaining continuouscommunication with mobile telephones moving from cell to cell. As amobile unit travels along a path that passes from one cell to another, ahandoff occurs which switches the mobile unit from a frequency in theset-assigned to the cell it is leaving, to a-new frequency in the setassigned to the cell it is entering. The handoff action is controlled bya mobile telephone switching office (MTSO) which receives a handoffcommand or instruction. The handoff command is typically generated whenthe signal received from the mobile telephone falls below a preselectedsignal strength thus indicating that the mobile telephone is at the cellboundary.

[0004] In an analog system, each cell in a cellular telephone systemoperates with a different assigned set of transmission frequencies. As amobile telephone passes from one cell to another, the handoff signalinstructs the cell which the mobile telephone is entering to begintransmitting at a frequency which is different from the frequency whichwas being transmitted by the cell which the mobile telephone wasleaving. A similar procedure is followed when the mobile telephonepasses into the next contiguous cell. Sets of assigned frequencies aredifferent for adjacent cells, and such sets are not repeated except forcells that are far enough away from each other so that interferenceproblems will not occur. In the case of systems using identificationcodes, the identification codes are generally not repeated.

[0005] A mobile telephone unit typically contains a control unit, atransceiver, and an antenna system. Each cell site typically is providedwith a control unit, radio, a power plant, data-terminals, and antennas.The MTSO provides coordination for all the cell sites and containssuitable processing and switching means. The MTSO also interfaces withthe telephone company zone offices for standard hardwired telephonesystems. The communication links between the MTSO and the various cellsites are typically microwave, T carriers, or optical fiber, and carryboth voice and control data between the cell sites and the MTSO.

[0006] When a user sitting in a car activates the receiver of the mobileunit, the receiver scans a plurality of set-up channels which aredesignated among the total channels assigned to the cell. Typically,there may be 21 set-up channels out of a total of 416 channels. (Theremainder are communication channels.) The receiver then selects thestrongest set-up channel and locks on for a certain time. Each site isassigned a different set-up channel. Accordingly, locking onto thestrongest set-up channel usually means selecting the nearest cell site.This self-location scheme is used in the idle stage and isuser-independent. It has a great advantage because it eliminates theload on the transmission at the cell site for locating the mobile unit.The disadvantage of the self-location scheme is that no locationinformation of idle mobile units appears at each cell site. Therefore,when the call initiates from a standard non-mobile or land line to amobile unit, the paging process is longer. Since a large percentage ofcalls originates at the mobile unit, the use of self-location schemes isjustified. After a delay, for example, one minute, the self-locationprocedure is repeated.

[0007] To make a call from a mobile unit, the user places the callednumber into an originating register in the mobile unit, checks to seethat the number is correct, and pushes a “send” button. A request forservice is-sent on a selected set-up channel obtained from aself-location scheme as described above. The cell site receives it, andin directional cell sites, selects the best directive antenna for thevoice channel to use. At the same time the cell site sends a request tothe MTSO via a high-speed data link. The MTSO selects an appropriatevoice channel for the call, and the cell site acts on it through thebest directive antenna to link the mobile unit. The MTSO also connectsthe wire-line party through the telephone company central office.

[0008] When a land-line party dials a mobile unit number, the telephonecompany central office recognizes that the called number is mobile andforwards the call to the MTSO. The MTSO sends a paging message tocertain cell sites based on the mobile unit number and a suitable searchalgorithm. Each cell site transmits the page on its own set-up channel.The mobile unit recognizes its own identification on a strong set-upchannel, locks onto it, and responds to the cell site. The mobile unitalso follows the instruction to tune to an assigned voice channel andinitiate an audible signal to alert the user to the incoming call.

[0009] When the mobile user is finished with the call, the hang up turnsoff the transmitter, and a particular signal (signaling tone) transmitsto the cell site and both sides free the voice channel. The mobile unitresumes monitoring pages through the strongest set-up channel.

[0010] During a call, two parties are on a voice channel. When themobile unit moves out of the coverage area of a particular cell site,the reception becomes weak. The present cell site requests a handoff viaan appropriate signal, for example, a 100 ms burst on the voice channel.The system switches the call to a new frequency channel or a differentcell identification code in a new cell site without either interruptingthe call or alerting the user. The call continues as long as the user istalking. The user does not notice the handoff occurrences.

[0011] When call traffic in a particular area increases, increasedcapacity may be generated by reducing the area covered by a particularcell, i.e., creating a microcell. For example, if a cell is split intofour smaller cells, each with a radius of one-half the original, trafficis increased four fold. Naturally, the smaller the cell, the morehandoffs required in a cellular telephone system for a given capacity.

[0012] Although in the proper circumstances, reduced cell size isadvantageous, certain problems can arise. Very often when cell size isreduced, for example to a radius of less than one mile, very irregularsignal strength coverage will result. This may be caused by buildingsand other structures, and can therefore become highly dependent upon thelocation of the mobile unit. Other problems arise in connection withsignal interference. Although some cellular telephone systems haveemployed several sets of frequencies in a small single cell, in anattempt to improve capacity in that cell, this prevents the reuse of thesame frequencies or adjacent frequencies in the neighboring cells. Theoverall capacity of the system thereby decreases, since the number ofavailable channels in a system is proportional to the inverse of thenumber of different frequency sets employed.

[0013] A cellular telephone system in which an antenna set configurationleads to a more uniform signal coverage contour and lowered interferencelevels is described in U.S. Pat. No. 4,932,049 issued to Lee. Thecellular telephone system comprises cells which contain a plurality ofantenna sets arranged and configured to limit propagation of signalssubstantially to one of a plurality of zones or sectors within theboundaries of the cells. The zones or sectors are substantially less inarea than the area of the cell. Transmission at any one frequency (ofthe assigned set of transmission frequencies for the cell) is confinedto the zone or sector wherein the mobile telephone has been assigned tosuch one frequency. Frequency handoff occurs while the mobile unit movesto a different cell.

[0014] In order to optimize the usage of the assigned set oftransmission frequencies in a zoned or sectored cell described above,multiple access schemes allowing more than one user to use acommunication channel could be implemented in the cell. Multiple accessis possible because most users of a voice communication system do notfully utilize the capacity of the communication system. Morespecifically, a typical user who is allocated a channel in thecommunication system only actively uses the voice channel for a fractionof its allocated time. As an example, a typical user using a voicechannel generally speaks for half of the time and listens for theremaining times. Thus the communication channel is then left unused forat least half of the time. By appropriate identifying by user time slotor code, bursts or pockets of voice signals for different users indigital systems may be transmitted thereby increasing the user capacityof the system.

[0015] Analog multiple access schemes such as analog frequency divisionmultiple access have been implemented in cellular telephone systems.Digital multiple access schemes including digital frequency divisionmultiple access, time division multiple access, and code divisionmultiple access have been developed, and it is anticipated that theywill also be implemented in cellular telephone systems. It isadvantageous to implement a multiple access scheme using digital means.This is because digital communication typically offers betterperformance, higher capacity, and lower cost. It should be noted thatthe applications of digital communication are not limited tocommunicating digital data. Analog voice signal can enjoy the benefitsof digital communication by first converting the analog voice signal toa digital signal before transmission. After the digital signal isreceived by a receiver, the digital signal is then converted back to theanalog voice signal.

[0016] One of the reasons for the improved performance in a digitalcommunication system is that the system is more tolerant to noise. Thisis because a threshold level of noise energy is required to change thestate of a digital signal. Thus, the communication is relatively errorfree if the noise energy of the communication medium is below therequired level. In addition, it is possible to implement error detectionand correction algorithms which further reduce the error rate even ifthe communication medium is relatively noisy. As a result, it ispossible to set up communication channels under noisy environmentthereby increasing the capacity of the communication system.

[0017] Another reason for the improved performance is that digital datacan be easily manipulated using digital processors. Thus, manyoperations which are difficult to implement using analog means can beimplemented using low cost microprocessors and digital logic circuits.

SUMMARY OF THE INVENTION

[0018] In accordance with the invention, an improved cell configurationleads to a more uniform signal coverage contour, lowered interferencelevels, increased capacity, improved voice quality, and relativelysimple and economical construction. The improved cell includes a mastersite and a plurality of zone sites. The improved cell also includes aplurality of antenna sets, each set being suitably positioned within theperiphery of the cell to cover a corresponding zone- and havingtransmitting and receiving means directionally configured to limitpropagation of signals substantially to a zone within the boundaries ofthe cell.

[0019] In the CDMA system according to the present invention, a uniqueidentification code is assigned to a mobile telephone located in thecell. A signal having a unique identification code is generated foridentifying the mobile telephone. The signal is coupled to the zones. Acombiner is also provided for combining signals from all of the zones inthe cell. A receiver is coupled to the combiner for retrieving thesignals having the code. According to another aspect of the invention,the signal coupled to the zones is delayed so that the transmission ofthe signal among the plurality of antenna sets is delayed by apreselected amount so as to reduce interference caused by successivereception of signals by the mobile telephone located in the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic diagram illustrating a typical layout of acell employed in the invention.

[0021]FIG. 2 is a schematic diagram illustrating another layout of acell employed in the invention.

[0022]FIG. 3 is a schematic block diagram of the electronics of anembodiment of the present invention.

[0023]FIG. 4 is a schematic block diagram of a zone select transmittersystem according to the present invention.

[0024]FIG. 5 is a schematic block diagram of an embodiment of a zonesite selector according to the present invention.

[0025]FIG. 6 is a schematic block diagram of another embodiment of azone site selector according to the present invention.

[0026]FIG. 7 is a schematic block diagram of a scanning receiver systemaccording to the present invention.

[0027]FIG. 8 is a schematic of a master site according to the presentinvention wherein set up channel is transmitted and received throughzone sites.

[0028]FIG. 9 is a schematic diagram illustrating a typical layout of acell in a code division multiple access (CDMA) system according to thepresent invention.

[0029]FIG. 10 is a schematic block diagram of the electronics in a CDMAsystem according to the present invention.

[0030]FIG. 11 is a schematic block diagram of the CDMA system includinga delay module according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] There are three main types of digital multiple access systems:digital frequency division multiple access (digital FDMA), time divisionmultiple access (TDMA) and code division multiple access (CDMA). Thepresent invention allows the implementation of digital FDMA, TDMA, andCDMA in a microcell. Thus, the benefits of digital multiple accesscommunication can be realized in the microcell. In addition, the presentinvention relating to the digital frequency division multiple access canalso be applied to an analog frequency division multiple access systemby a person of ordinary skill in the art.

[0032] Frequency division multiple access, both analog and digital is amethod whereby the bandwidth of a communication channel is subdivided infrequency into subchannels so that more than one user can use thecommunication channel simultaneously.

[0033] TDMA is a method whereby the time of operation of a communicationsystem is divided into a plurality of time slots having predeterminedlengths. The system transmits information relating to a user only duringthe time slots assigned to the user. The system has means to temporarilystore -information generated by the user during other times so thatinformation is not lost during these times. Thus, more than one user isable to use the same channel in the communication system.

[0034] In a preferred cellular TDMA system, the bandwidth of a channelis 30 KHz. Three callers have access to a particular channel.Communication time is divided into time frames of 20 ms and the timeframes are divided into three slots of 6.66 ms each. Each mobile unit isassigned one of the three time slots in a particular channel.

[0035] CDMA is a method whereby each user is assigned a different codingscheme instead of being assigned a different frequency channel or adifferent time slot. These coding schemes are orthogonal or partiallycorrelated to each other so that it is possible to identify the userbased on an analysis of the codes used in the transmission. As a result,more than one user can use the same channel.

[0036] CDMA is especially desirable if the communication channel isrelatively noisy. This is because CDMA typically uses spread spectrumtechniques which are known to be tolerant to noise and multipathinterference. As a result, CDMA allows more users to use the noisychannel to make initial calls thereby increases the capacity of thechannel.

[0037] Another advantage of CDMA is that every cell uses the same set ofwide band frequencies, or channels. As a result, it is possible to havethe closest co-channel separation, i.e., the ratio of the co-channelseparation distance (D) and the cell radius (R) in a CDMA system couldbe equal to 2, whereas the ratio D/R for other communication methods isabout 4.6.

[0038] A consequence of using the same set of wide band frequencies inevery cell is that no frequency switching is required as mobile unitsmove from cell to cell. Instead, the coding scheme in CDMA has a codefor identifying the cells. As mobile units move from cell to cell, onlythe identification codes for the cells need to be changed. Such changein the identification code instead of changing frequency is referred toas “soft” handoff. As a result, the performance of the system improves.

[0039] An example of a microcell in which the system according to thepresent invention can be used is shown in a cell 1 of FIG. 1. Thestructure of cell 1 is disclosed in U.S. Pat. No. 4,932,049 issued toLee, which is incorporated herein by reference. This structure leads toa more uniform signal coverage contour and lowered interference levels.

[0040] The outer boundary of cell 1 is delineated by the circle 11 insolid line. Although shown as a circle, cells are often represented ashexagons in designed illustrations. In reality, however, due to theshape of terrain and the presence of buildings and other structures, theactual boundary of the circle 11 may be of an irregular shape. In anycase, the solid line 11 is intended to represent that location at whicha mobile telephone unit passes from the influence of the illustratedcell and into the influence of an adjacent cell.

[0041] Three separate antenna sets 13, 15, and 17 are positioned withincell 1. Antenna set 13 is located at a zone site 14, whereas antennasets 15 and 17 are located at zone site 16 and 18, respectively. One ofthe zone sites, for example, zone site 14, can collocate with a mastersite which processes the signal to and from the zone sites. Dependingupon the particular conditions within the cell area, other numbers ofantenna sets may be usefully employed, and it is to be understood thatthe use of three sets in FIG. 1 is for illustrative purposes only. Eachantenna set includes a transmitting antenna 13 a, 15 a, and 17 a,respectively. Each antenna set also includes two receiving antennas 13 band 13 c, 15 b and 15 c, and 17 b and 17 c, respectively. Duplication ofthe receiving antennas at each zone site is for diversity use to reducesignal fading by combining the signals. The determination of thelocations of zone sites and the number of zone sites in a cell can bebased on the Lee's coverage prediction model published in IEEETransactions on Vehicular Technology, February, 1988.

[0042] Each antenna set has its own zone of major influence fortransmitting and receiving signals. Thus, antenna set 13 at the mastersite 14 has a zone indicated by the dotted line 13 z. Similarly, antennaset 15 at zone site 16 has a zone of influence designated by the dottedline 15 z and antenna set 17 at the zone site 18 has a zone of influencedesignated by the dotted line 17 z. It may be seen from FIG. 1 that thezones overlap in certain areas. Directionality is provided to theantenna sets so that the zones of influence, i.e. the zones ofpropagation and reception of signals, are limited to be substantiallywithin the boundaries of cell 1. Such directionality is provided bysuitable means such as shown as a symbolic means 19 arranged at eachantenna set or zone site. The directionality means 19 can be a reflectorfor each individual antenna, or any other suitable arrangement toprovide the desired directionality and coverage.

[0043] The signal of the set up channels can be communicated to themobile units inside cell 1 in two different configurations. The firstconfiguration uses an additional antenna set in the master site, in thiscase, zone site 14. Thus, antenna set 13 includes a setup transmittingantenna 13 d for transmitting set up signals, and duplicate setupreceiving antennas 13 e and 13 f for receiving set up signals, as willbe explained below. The set of setup antennas 13 d, 13 e and 13 f,however, is configured to have a greater zone of influence, this beingindicated by the dash-dot line 21, substantially coextensive with thelimits of cell 1 delineated by circle 11. The second configuration usesthe same sets of antenna 13 a-c, 15 a-c, and 17 a-c for communicatingboth the voice channel and set up channel. In this configuration, noadditional antenna set is required.

[0044]FIG. 2 is another example of microcell 110 in which the systemaccording to the present invention can be used. Microcell 110 ispreferably positioned along a highway 150 for providing cellulartelephone services to mobile units (not shown) moving along highway 150.Microcell 110 comprises a plurality of zones, illustrated by the dottedcircles 113-116. It is to be understood that the number of zones in FIG.2 and the shape of the zones are for illustrative purposes only. Eachzone comprises a zone site for housing at least one antenna set. Thuszones 113-116 comprises zone sites 123-126 and antenna sets 133-136. Oneof the zone sites can also be a master site.

[0045] The advantage of microcell 110 is that a long stretch of highway150 can be covered by a set of assigned frequencies. Thus, a mobile unitcan travel a long distance without the need for a handoff action. Inaddition, the power radiated by antennas 133-136 could be low and stillcover the stretch of highway because the area of each zone is small. Asa result, the microcell 110 could be implemented using low costequipment.

[0046]FIG. 3 shows a block diagram of the electronics which can be usedeither in a TDMA or in a digital FDMA located in the cell of FIG. 1. Twozone sites 16 and 18 are each coupled to a master site 14 and arecontrolled therefrom. In the illustrated embodiment, zone site 16 isconnected to master site 14 via three cables 23, 25, and 27. Zone site18 is connected to master site 14 via cables 29, 31, and 33. Thespecific nature of the signals assigned to the respective cables will bedescribed below. Generally, however, cables 23 and 29 carry transmitterantenna signals whereas cables 25, 27, 31, and 33, carry receiverantenna signals. The illustrated embodiment depicts the communicationbetween the zone sites and the master site as being via cable. It willbe apparent to those skilled in the art that such cables may include,for example, T1 carrier cables, optical fibers, or the like. The cablesmay also be replaced by microwave channels.

[0047] The zone sites each contain a signal processing ensemble ofcomponents as shown at 35 for zone site 14 a. It is understood thatsubstantially identical signal processing ensembles are contained inzone sites 16 and 18, although such ensemble are not shown in FIG. 3 forsimplicity signal processing ensemble 35 includes a filter 37, anamplifier 39, and a converter 41 interposed between antenna 13 a andoutput cable 43. Similarly, filter 45, amplifier 47, and converter 49are interposed between antenna 13 b and output cable 51, and filter 53,amplifier 55, and converter 57 are interposed between antenna 13 c andoutput cable 59. The filters, amplifiers, and converters filter,enhance, and convert signals as desired and may be of any type suitablefor the stated purpose.

[0048] In FIG. 3, the three amplifiers 39, 47, and 55 enhance the UHFsignals applied to their input from filters 37, 45, and 53 respectively.These UHF signals are then applied to converters 41, 49 and 57, whicheither up convert or amplitude modulate the frequency to an opticalfrequency, where optical fibers are used for the cable connections, ordown convert the frequency to a base band for passing through T1 carriercables. They may also directly convert from-UHF to microwave wheremicrowave channels are used. The filters, amplifiers and converters maybe of any type suitable for the stated purpose.

[0049] Master site 14 comprises a zone selector 95, a transmitter module96, a receiver module 97, a controller 98, and a set up channel 99.Controller 98 communicates with the MTSO. Transmitter module 96comprises a plurality of transmitters. Each transmitter generates asignal having a frequency corresponding to the assigned frequency of achannel. The signals generated from the transmitters in transmittermodule 96 is coupled to the appropriate zone site through zone selector95 for communication with the mobile units. Zone selector 95 alsoreceives signals from the three zone sites, and, after processing thesesignals in a manner described below, couple the signal to receivermodule 97. Receiver module 97 comprises a plurality of receivers forrecovering the signals generated by mobile units in the cell. Eachreceiver is a two-branch diversity receiver, well known in the art,which comprises two inputs, each input accepting a signal from one ofthe two receiving antenna in the zone site. Each receiver is tuned to afrequency corresponding to the assigned frequency of a channel. Therecovered signals are coupled to controller 98.

[0050] Zone selector 95 comprises a zone switch 92, a zoneswitch/combiner 94, and a zone scanner 93. Zone switch 92 receivessignal from transmitter module 96 and directs the signal to theappropriate zone site for communication with the mobile units. Anexemplary implementation of zone switch 92 is shown at FIG. 4. Theselection of the appropriate zone site is determined by a selectionsignal generated by zone scanner 93. An exemplary implementation of zonescanner 93 is shown at FIG. 7. Zone switch/combiner 94 receives signalfrom the zone sites, and, depending on the mode of operations, describedbelow, either combines the signals from the three zone sites or selectsa signal from one zone before coupling the resulting signal to receivermodule 97. Exemplary implementations of zone switch/combiner 94 areshown at FIGS. 5 and 6.

[0051] At master site 14, the output ports 71-73 of zone switch 92 gothrough converters 61-63, respectively, and then coupled to zone sites14 a, 18 and 16, respectively, through cable connectors 43, 29, and 23,respectively. The selection of the appropriate zone site is determinedby a selection signal generated by a zone scanner which is input to zoneswitch 92 through an input port 87. Zone switch 92 also has an inputport 88 for inputting a set of transmitting signals generated bytransmitter module 96.

[0052] Zone scanner 93 comprises three input ports 81-83 for couplingsignals from the three zone sites via converters 64-66, respectively.The strength of these signals are compared to determine the zone sitewhich gives rise to the strongest signal. Alternatively, the zone sitecan also be selected based on the supervisory audio tone (SAT) signal.Zone scanner 93 also comprises an input port 85 for accepting a timedivision clock signal from receiver module 97 for separating theappropriate time slot. The selection signal generated by zone scanner 93is sent to an output port 84 for coupling to zone switch/combiner 94 andzone switch 92.

[0053] The signal received from the three zone sites, after goingthrough cable connectors 25, 27, 31, 33, 51, 59, and converters 64-69,terminates at the input ports 74-79 of zone switch/combiner 94. If zoneswitch/combiner 94 operates in a zone switching mode, a selection signalis coupled to an input port 89. The selection signal selects one of thesignals from one of the three zone sites for coupling to the outputports 90, 91. If zone switch/combiner 94 operates in a combing mode suchthat the signals from all the zones are combined, the selection signalis not used. Zone switch/combiner 94 generates two sets of outputsignals, one at an output port 90 and the other at an output port 91.Each member of each set of output signals is coupled to a correspondinginput terminal of a two-branch diversity receiver in receiver module 97.

[0054] It can be understood by a person of ordinary skill in the artthat if master site 14 is co-locate with one of the zone site, say zonesite 14 a, no converter is required for communicating between mastersite 14 and the co-located zone site 14 a. In this case, converters 41,61, 49, 64, 57, and 67 are not needed.

[0055] Controller 98 measures the signal strength of a channel requestedby the MTSO. If the initial call is in this particular cell, or if thecall is handed off to this particular cell through the controller, thecontroller initiates one of the transmitters in transmitter module 96 totransmit at a particular frequency assigned by a MTSO to that call. Thesignal is then sent to a proper zone through zone switch 92. If duringthe call, the signal strength received at controller 98 is below apreselected level, the controller initiates a handoff process from theMTSO to handoff the call to another cell.

[0056] In FIG. 3, controller 98 is connected to a set up channel 99which transmits and receives signals on the three control antennas 13 d,13 e, and 13 f. The set up channel assigned in each cell can cover theentire zone of influence 21, shown in FIG. 1, which is coextensive withcell 1 in FIG. 1. However, it is also possible to transmit the set upchannel 99 to each zone site so that no setup channel antennas areneeded. In this case, all zone sites transmit and receive the setupchannel inside its zone of influence. An exemplary setup channel whichdoes-not use setup channel antennas is shown in FIG. 8.

[0057] In operation of the system above described, a mobile unit whichis operating on an assigned frequency f₁ in the cell will typically bemoving within the cell. All zone sites within the cell will receivesignal levels (strengths), but only that zone site at which the receivedsignal level is the strongest will transmit and receive signals to themobile unit during a call. The transmitters in the other zone sites donot transmit to the mobile unit. When the mobile unit moves such thatthe received signal strength at a zone site other than the one that iscurrently transmitting becomes strongest, the system operates to turnoff the- transmitter at the weaker zone site and turn on the transmitterat the zone site at which the stronger signal level is being received.The two-diversity antenna signal at each zone are also selected from theproper zone site to receive the call. The operating frequency, however,remains unchanged at f₁. Thus, no handoff has occurred in thetraditional sense and the MTSO is not involved. As a result, noadditional handoff load is added to the MTSO switching equipment. Analternative way is to combine the two diversity antenna signals from allzones, as described below.

[0058] As was noted above, the block diagram shown in FIG. 3 can be usedboth in digital FDMA and TDMA. In a TDMA scheme, a plurality of timedivision multiplexers and an associated synchronization clock is used,as explained below. In a digital FDMA scheme, it is not necessary to usethe time division multiplexers and the associated clock.

[0059]FIG. 4 shows a block diagram of an exemplary zone switch, shown asnumeral reference 92 in FIG. 3, according to the present invention. Zoneswitch 200 comprises two input port 283, 285 and three output ports211-213 which correspond to ports 87, 88, and 71-73, respectively, ofzone switch 92 in FIG. 3. Thus, signals from a transmitter module, shownas 96 in FIG. 3, is coupled to input port 285 of zone switch 200. Thesesignals are directed by zone switch 200 to the three zone sites 14 a,18, and 16 through output ports 211-213.

[0060] As was noted above, transmitter module 96 comprises a pluralityof transmitters, each generating a different signal. Thus, Input port285 further comprises a plurality of input terminals, shown as 286 and287 in FIG. 4. Terminal 286 couples a signal having a frequency of f₁into zone switch 200 and terminal 287 couples a signal having afrequency of f₂ into transmitting zone switch 200.

[0061] Zone switch 200 further comprises a plurality of time divisionmultiplexers, two of them, 251 and 261 are shown at FIG. 4 forillustrative purpose. In general, the number of time divisionmultiplexers is the same as the number of frequency channels assigned tothe cell. Zone switch 200 also comprises a plurality of channel zoneswitches, six of them, 241-246, are shown at FIG. 4. In general, thenumber of zone switches is equal to the product of the number of timeslots and the number of time division multiplexers. Zone switch 200further comprises three combiners 221-223, one associated with each zonesite, for combining the signals from the channel zone switches forsending to the three zone sites.

[0062] A time division multiplexer (TDM) is a device, well known in theart, for dividing time intervals into time slots. In FIG. 4, TDM 251 and261 divide each time interval into three time slots. Preferably eachtime slot is 6.66 ms long in a 20 ms time interval. It may be understoodthat a TDM can divide time intervals into any suitable number of timeslots, and the choice of the number of divisions in TDM 251 and 261 arefor illustrative purposes only.

[0063] TDM 251 comprises an input port 253 for accepting signals havinga frequency of f₁, generated by a transmitter of transmitter module 96,shown in FIG. 3. The time interval for transmitting the signal havingfrequency f₁ is divided into three time slots. The signals of the timeslots are coupled to output ports 256-257. Each of these signals iseventually directed to a zone site for communicating with a mobile unit.Since different time slots can be directed to different zone sites, itis possible that three mobile units in three zone sites communicate withmaster site 14 using the same frequency channel.

[0064] Similarly, TDM 261 comprises an input port 263 and three outputports 266-268. Ports 263 and 266-268 correspond to ports 253 and256-258, respectively, of TDM 251. TDM 261 functions in a similar way asTDM 251. All the output signals from TDM 261 have frequency f₂ since theinput signal to TDM 261 has a frequency of f₂. Again, signals havingfrequency f₂ in the three time slots can be directed to the same ordifferent zone sites.

[0065] Each of the output ports from the TDMs is coupled to a channelzone switch for directing the output of a communication channel from theTDMs to the appropriate zone site. Thus, output ports 256-258 and266-268 are coupled to channel zone switches 241-246, respectively. Theconstruction of all the channel zone switches are substantially thesame. Thus, only one channel zone switch, 241, is described in detail.

[0066] Channel zone switch 241 comprises an input port 231 for acceptingsignals from TDM 251. Channel zone switch 241 also comprises threeoutput ports 235-237 coupled to combiners 221-223, respectively, fordirecting signals to one of the three combiners. Channel zone switch 241further comprises a switch 233 for selectively coupling the input port231 to one of the three output ports 235-237. The coupling is controlledby a selection signal at a control port 232. Control port 232 is coupledto input port 283. As was noted above, input port 283 corresponds toport 87 in FIG. 3 which is coupled to zone scanner 93. Depending on thestatus of the signal at input port 232, the signals from output port 256of TDM 251 could be sent to one of the three zone sites.

[0067] Similarly, each channel zone switch has three output ports forcoupling to the three combiners. Again, depending on the status of thecontrol port, the outputs of the channel zone switch is coupled to oneof the three combiners. Each of the combiners 221-223 combines all thesignals coupled thereto and sends the signals out to the zone sitesthrough output ports 211-213, respectively.

[0068] As was noted above, a typical TDMA system divides a time intervalof 20 ms into time slots. If quadruture phase shift-keying modulation isused, a total of 486 symbols can be transmitted within the 20 ms timeinterval, i.e., the time duration for each symbol is 41 microseconds. Inorder to ensure that the last symbol of one slot and the first symbol ofthe next slot are correctly received, the rate of switching should befaster than the time duration for a single symbol, i.e., 41microseconds. In order to prevent undesirable effects resulting fromswitching transients, a TDM switch which has a switching ratesubstantially faster than 41 microseconds should be used. Examples ofsuch TDM switches are part numbers 54F/74F 151A manufactured by NationalSemiconductor and 10G050A manufactured by GBL.

[0069] It should be noted that the block diagram shown in FIG. 4 canalso be used in a digital FDMA scheme if the TDMs are removed from theblock diagram. In this case, the signals from the transmitters intransmitter module 96, shown in FIG. 3, are coupled directly to thechannel zone switches, and the number of channel zone switches are thesame as the number of transmitters instead of three times the number oftransmitters if the TDMs are included. Thus, terminal 286 couples asignal having frequency f₁ to one of the three channel zone switches241-243. Similarly, terminal 287 couples a signal having frequency f₂ toone of the three channel zone switches 244-246.

[0070]FIG. 5 is a block diagram of a zone switch/combiner 300 accordingto the present invention. In this embodiment, zone switch/combiner 300operates as a combiner and will be referred to as zone combiner in thefollowing description of FIG. 5. The block diagram in FIG. 5 alsoincludes a receiver module 330. Zone combiner 300 and receiver module330 correspond to zone switch/combiner 94 and receiver nodule 97 in FIG.3.

[0071] Zone combiner 300 comprises seven input ports 303, 304-309, andtwo output ports 301, 302. Input ports 303, 304-309 and output ports301, 302 correspond to input ports 89, 74-79 and output ports 90, 91,respectively, of FIG. 3. Thus, the signals at input ports 304-309 aresignals from the zone sites.

[0072] The signals from input ports 304-306 are combined by combiner 321and coupled to output port 301. Since input ports 304-306 are coupled tothe zone sites, it means that all three signals from all the three zonesites are combined together by combiner 321. Similarly, the signals frominput ports 307-309 are combined by combiner 321 and then coupled tooutput port 302. Again, input ports 307-309 are coupled to the threezone sites, thus, the three signals from the three zone sites arecombined together by combiner 321. The signals at output ports 301 and302 are coupled to receiver modules 330 in an arrangement describedbelow. Since there is no need to select the zone sites in thisembodiment, the select signal present at port 303 is not used.

[0073] Receiver module 330 comprises a plurality of two-branch diversityreceivers, only three of these receivers, 333, 335, 337, are shown inFIG. 5. These receivers 333, 335, 337 are TDM receivers and couldrecovet individual signals sent by the mobile units. Each receiver inmodule 311 is tuned to a frequency corresponding to the frequencygenerated by a corresponding transmitter in transmitter module 96, shownin FIG. 3.

[0074] Each receiver in receiver module 330 comprises two input ports.One of the input port is coupled to port 301 and the other input port iscoupled to port 302. The recovered signal from each receiver is sent tocontroller 98, shown in FIG. 3. It is well known in the art that thetwo-branch diversity receiver arrangement enhance the quality of thereceived signal.

[0075] Since all the receivers in receiver module 330 are TDM receivers,all the receivers share a common clock (not shown) which can be used forsynchronization with the time slots of the TDMs in zone switch 200,shown in FIG. 4. The common clock signal is sent out of receiver module330 through an output port 340. As was noted above, this clock signal iscoupled to zone scanner 93, shown in FIG. 3, for synchronization.

[0076] It should be noted that if the block diagram of FIG. 5 is used ina digital FDMA scheme, no synchronization clock is needed. Consequently,port 341 is not needed.

[0077] Zone combiner 300, shown in FIG. 5, is preferably used if thetransmission rate is low, typically less than 10 kilobits per second, orthe distance between the zone sites and the master is short, typicallyless than one half of a kilometer. Otherwise, another implementation ofzone site selector, shown at FIG. 6, should preferably be used.

[0078] Even though zone combiner 300 is described as part of a TDMA anda digital FDMA scheme, it should be noted that zone combiner 300 canalso be used in an analog multiple access system and in a portabletelephone system. One of the advantages in using zone combiner 300 in ananalog system is that the power delivered to the receivers in receivingmodule 330 is increased because all the power from the three zone sitesare utilized. Another advantage is that temporal loss of received signalfrom one zone would have less effect on the quality of the signalbecause signals from the other zones could compensate for such loss.

[0079]FIG. 6 is a block diagram of a receiver module 390 and a zoneswitch/combiner 350 which is suitable for high transmission rates or insituations where the distance between the zone sites and the master siteis long. In this embodiment, zone switch/combiner 350 operates as a zoneswitch, and will be referred to as a zone switch for receiving in thedescription of FIG. 6. Receiver module 390 is similar to receiver module330 of FIG. 5 and comprises a plurality of two branch diversityreceivers 391-393 for recovering signals transmitted by mobile units.Receiver module 390 also comprises an output port 394 for sending aclock signal to zone scanner 93, shown in FIG. 3, for synchronization.

[0080] Zone switch 350 comprises two sets of channel zone switches 360and 365. Zone switch 350 further comprises two output ports 352, 353,and seven input ports 351, 354-357 which correspond to ports 90, 91, 89,and 74-79, respectively, of zone switch/combiner 94, shown in FIG. 3.Input signals from input ports 357-359 are coupled to the first set ofchannel zone switches 360. Input signals from input ports 354-356 arecoupled to the second set of channel zone switches 365.

[0081] Each set of channel zone switches 360, 365 has a plurality-ofchannel zone switches, 361-363 and 366-368. The number of channel zoneswitches in each set is the same as the number of channels in receivingmodule 390. Each channel zone switch selects one of the zone sites inresponse to a control signal at input port 351. Since input port 351corresponds to port 89 in FIG. 3, the control signal is a signal fromzone scanner 93, shown in FIG. 3.

[0082] The channel zone switches in zone switch 350 for receiving issubstantially the same as the channel zone switches in zone switch 200for transmitting, shown in FIG. 4, except that the channel zone switchesin receiving zone switch 350 have three input ports and one output portinstead of three output ports and one input port. Again, a selectionsignal is used to determine the coupling of the output port to one ofthree input ports. Since the operations of all the channel zone switchesare the same, only one channel zone switch, 361, is described in detail.

[0083] Channel zone switch 361 comprises three input ports 371-373 foraccepting signals from input ports 359, 358, and 357, respectively.Channel zone switch 361 also comprises an output port 375 for couplingsignal to a terminal 381 inside output port 353, preferably an outletbox, of receiving zone switch 350. Channel zone switch 361 furthercomprises a switch 376 for selectively coupling the output port 375 toone of the three input ports 371-373. The coupling is controlled by aselection signal at a control port 374. Control port 374 is coupled toinput port 351. As was noted above, input port 351 corresponds to port89 in FIG. 3 which is coupled to zone scanner 93. Depending on thestatus of the signal at input port 375, the signals from one of theinput ports 371-373 of channel zone switch 361 could be sent to outputport 375.

[0084] The output signal from channel zone switch 361 is coupled to aninput port of receiver 391. This signal comprises one branch of atwo-branch diversity signal. Similarly, the output from channel zoneswitch 366 is coupled to another input port of receiver 391. This signalcomprises a second branch of a two-branch diversity signal. Receiver 391recovers the signal transmitted by a mobile unit and sends this signalto controller 98, shown in FIG. 3.

[0085] Similarly, the output signals from channel zone switches 362, 367are coupled to receiver 392 and the output signals from channel zoneswitches 363, 368 are coupled to receiver 393. The signals recovered byreceivers 392, 393 are coupled to controller 98.

[0086] It should be noted that if the block diagram of FIG. 6 is used ina digital FDMA system, no synchronization clock is needed. Consequently,port 341 is not needed.

[0087]FIG. 7 is an exemplary implementation of a zone scanner 400.Scanning receiver 400 comprises three frequency scanners 411-413, threetime slot switches 421-423, and a comparator 427. Zone scanner 400further comprises four input ports 437, 431-433 and an output port 439.Ports 431-433, 437, and 439 correspond to ports 81-83, 85, and 84,respectively, of zone scanner 93, shown in FIG. 3.

[0088] Signals from zone sites 14 a, 18, and 16 are coupled to frequencyscanners 411-413 through input ports 431-433, respectively. Frequencyscanners 431-433 scan a predetermined number of frequencies from zonesites 14 a, 18, and 16, respectively. Time slot switches 421-423selectively couple one of the three scanners 411-413 to comparator 427at any given time. The timing for coupling one of the three scanners411-413 is controlled by a clock signal input from port 437.

[0089] Comparator 427 stores and compares the average signal strength ofthe signals from the three zone sites. As a result, it is possible todetermine the zone site which gives rise to the strongest signalreceived at the master site. This information is coupled to output port439 as a selection signal for controlling the zone switches. Comparator427 preferably includes hysteresis means for reducing the ping pongeffects resulting from instantaneous signal fluctuations. Comparator canalso be used to compare the strongest supervisory-audio-tone signalsamong the three zones.

[0090] It should be noted that if the block diagram of FIG. 7 is used ina digital FDMA scheme, no synchronization clock and time slot switch isneeded. Consequently, time slot switches 421-423 and port 431 are notneeded.

[0091]FIG. 8 is a schematic block diagram of a master site 640 whereinthe setup channel is transmitted and received by the three zone sitesinstead of using setup channel antennas. The zone switch/combiner 94,zone switch 92 for transmitting, zone scanner 93, controller 98, andconverters 61-69 in FIG. 8 function the same and share the same numeralreferences as the corresponding elements of FIG. 3. Consequently, theseelements and their connections are not described here.

[0092] Controller 98 is coupled to a set up transmitter 612 which is inturn coupled to a power splitter 614. Power splitter 614 splits thesignal generated by set up transmitter 612 into three substantiallyidentical signals. Each of the three signals is coupled to acorresponding combiner 616, 618, 620. Signals from the output ports71-73 of zone switch 92 for transmitters are also coupled to combiners620, 618, and 616, respectively. The combined signals from combiners620, 618, and 616 are coupled to converters 61-63 for sending to thecorresponding zone sites. A power splitter is used in FIG. 8 because thelocation of the mobile unit is not known during set up operations.

[0093] Signals from converters 64-69 are coupled to a combiner 624 inaddition to zone switch/combiner 94. Combiner 624 combines the signalsfrom converters 64-66 into one signal and couples the combined signal toone input port of a two-branch diversity set up receiver 622. Combiner624 also combines the signals from converter 67-69 into one signal andcouples the combined signal to a second input port of receiver 622.Receiver 622 recovers the set up channel transmitted by a mobile unitand couples the signal to controller 98.

[0094] Zone switch/combiner 94 can either be of a type comprising acombiner, shown in FIG. 5, or of a type comprising channel zoneswitches, shown in FIG. 6. If zone switch/combiner 94 comprises acombiner, this combiner can be physically combined with combiner 624.

[0095] It should be noted that the arrangement in FIG. 8 can be used inanalog frequency division multiple access, digital frequency divisionmultiple access, TDMA, and CDMA.

[0096]FIG. 9 is a schematic diagram illustrating a typical layout of acell 500 utilized in a CDMA system according to the present invention.The outer boundary of cell 500 is delineated by a circle 511 in solidline. The circle is used for illustrative purposes only and the actualboundary of cell 500 may have an irregular shape. Three separate antennasets 521-523, are each positioned in a zone site 516, 514, and 518,respectively, within cell 500. A master site is co-located with a zonesite, in this case, zone site 514. Depending upon the particularconditions within the cell area, other members of antenna sets may beusefully employed.

[0097] Each antenna set includes a transmitting antenna 521 a, 522 a,and 523 a. Each antenna set also includes two receiving antennas 521 band 521 c, 522 b and 522 c, and 523 b and 523 c, respectively.Duplication of the receiving antenna at each sub-site is for diversityuse to reduce signal fading by combining the signals. Directionality ofthe antenna is provided by suitable means, shown as a symbolic means519, for each set of antennas. Each antenna set has its own zone ofmajor influence for transmitting and receiving signals. Thus antenna set521-523 has zones of influence designated by dotted lines 531-533,respectively. In contrast to the antenna arrangement shown in FIG. 1,there is no separate setup channel antennas.

[0098] In the CDMA system according to the present invention, the threezone sites are transmitting and receiving signals continuously. Thus,cell 500 becomes a three-zone microcell. Since the radius of eachmicrocell is about half that of the cell, the power level required isreduced by a factor of four. Consequently, the amount of interference toneighboring cells are reduced substantially thereby resulting in higherquality. In addition, the reduced power level also allows the use of lowcost equipment.

[0099]FIG. 10 is a schematic block diagram of the electronics of a CDMAsystem according to the present invention. The functions of thecomponents in FIG. 10 are substantially the same as the functions of thecomponents in FIG. 3, except that zone selector 95, which comprises zoneswitch 92, zone scanner 93, and zone switch/combiner 94, is replaced bya zone selector 582, which comprises a combiner 581. The componentshaving the same functions in FIGS. 3 and 8 are shown with the samenumeral references, and the functions and connections of thesecomponents are not described here.

[0100] The CDMA system, shown in FIG. 10, comprises a transmitter module573 which includes at least one wide-band (spread spectrum) transmitterfor generating signals having the appropriate codes at the initiation ofa signal from a controller 98. The signal generated by module 573 arecoupled to combiner 581. The combined signal is sent to all the zonesites for the antenna set inside the zone site. Unlike the TDMA system,it is not necessary to divide time intervals into time slots and selectthe appropriate zone sites.

[0101] Signals received by all the zone sites are also combined bycombiner 581. Thus, signals received by converters 64-65 are combined bycombiner 581. The combined signal is sent to one input port of all thetwo-branch diversity receivers in a receiver module 575. Similarly,signals received by converters 67-69 are combined by combiner 581 andsent to a second input port of all the two-branch diversity receivers inreceiver module 575. Receiver module 575 comprises at least one CDMAreceiver, well known in the art, for recovering the signals sent by themobile units to the master site 514. After the signals coupled to thereceivers are diversity combined, they are coupled to controller 98.

[0102] Refer now to FIG. 11. FIG. 11 is a schematic block diagram of theelectronics of a CDMA system according to the present invention with theaddition of a time delay module which serves to reduce interferencecaused by reception of multiple signals at differing times. Thefunctions of the components in FIG. 11 are substantially the same as thefunctions of the components in FIG. 10. The components having the samefunctions in FIG. 10 are shown with the same numeral references, and thefunctions of these components are not described here.

[0103] Those skilled in the art will recognize that land-based cellulartransmission experiences signal fading that typically consists of theRayleigh fading component with a direct N component. In the multipletransmitter arrangement of the present invention, there is an areawithin the preferred cell that falls within the zone of influence of allthree antenna sets designated by lines 531-533 in FIG. 9. As a result,multiple signals originating from three antenna sets arrive almostsimultaneously at the mobile receiver from many directions with manydifferent transmission delays. In most situations, the delay between thereceived signals will be large enough to allow a correllator, of aconstruction well-known to those skilled in the art (not shown), todifferentiate among and combine the signals. However, as the size of thecell shrinks, the delay between the signals becomes too small to allowthe correllator to function properly. At the UHF frequency bands usuallyemployed for mobile radio communications, including those of cellularmobile systems, significant phase differences in signal traveling ondifferent paths may occur. The possibility for destructive summation ofsignals may result.

[0104] In a CDMA cellular telephone system, high modulation allows manydifferent production paths to be separated, provided the difference inpath propagation delays exceed the modulation chip duration, or onebandwidth. As an example, when pseudo noise (PN) modulation is employedas the preferred modulation means, if a PN chip rate of one MHz is used,the full spread spectrum processing gain, equal to the ratio of thespread bandwidth to the system data rate, can be employed against pathsthat differ by more than one microsecond in path delay from the desiredpath. A microsecond path delay differential corresponds to differentialpath distance of 1,000 feet. The urban environment typically providesdifferential path delays in excess of one microsecond, and up to 1-20microseconds are reported in some areas.

[0105] In the instant invention, the signal transmitted by each antennaset is preferably a direct sequence spread spectrum signal modulated bya PN sequence clock at a predetermined rate, which in the preferredembodiment is 1.25 MHz. A property of the PN sequence as used in thepresent invention is that discrimination is provided against multi-pathsignals. When the signal arrives at the mobile receiver after passingthrough more than one path, there will be a difference in reception timeof the signals. This reception time difference corresponds to thedifference in distance divided by the speed of light. If this timedifference exceeds one microsecond, then a correlation process can beemployed to discriminate against one of the paths.

[0106] As discussed above, however, smaller cell size decreases thereception time difference. In order to ensure an initial time differencein excess of one microsecond between each of the three signals beingtransmitted from the exemplary antenna sets of the cell, delay module601 is advantageously employed to ensure the appropriate transmissiondelay between the signals being transmitted by antenna sets 13, 15 and17. Delay module 601 is constructed in a manner well-known to thoseskilled in the art. Delay module 601 is shown comprising time delaycircuits for each of the three zone sites. However, those skilled in theart will recognize that the number of required delay circuitscorresponds to the number of transmitting antenna, minus one.Corresponding delay modules 602 and 603 are employed to delay thesignals arriving from the zone cites. These modules allow correlationand combination of signals transmitted from the mobile unit in the cellto the three antenna sets. Those skilled in the art will recognize thatmodules 601-603 may be placed in other locations in the reception andtransmission lines and may include or work in conjunction withcorrelator and/or combiner modules.

[0107] Various modifications of the invention, in addition to thoseshown and described herein, will be apparent to those skilled in the artfrom the foregoing description and accompanying drawings. Suchmodifications are intended to fall within the scope of the appendedclaims.

What is claimed is:
 1. In a cellular telephone system wherein aplurality of contiguous cells, each having an assigned set ofidentification codes, are arranged with handoff means for maintainingcontinuous communication with a plurality of mobile telephones movingfrom cell to cell; a cell having a plurality of antenna sets, each setbeing positioned and having transmitting and receiving meansdirectionally configured to limit propagation of signals substantiallyto a zone within the boundaries of said cell; means for assigning atleast one of the identification codes of the assigned set thereof to atleast one mobile telephone located in said cell including means forgenerating a signal having a unique identification code of the assignedset thereof for identifying said at least one of said mobile telephones;means for coupling said signal to said zones; at least one combiner forcombining signals from all the zones in said cell; and a receivercoupled to said combiner for retrieving signals having said uniqueidentification code.
 2. A cell according to claim 1 wherein said meansfor coupling comprises means for splitting said signal and means fordirecting said splitted signal to said zones.
 3. A cell according toclaim 2 wherein said means for coupling further comprises time delaymeans for delaying the transmission of said signal among the pluralityof antenna sets by a preselected amount so as to reduce interferencecaused by successive reception of said signals by said at least onemobile telephone.
 4. A cell according to claim 1 which further comprisesmeans for transmitting a set up channel and means for receiving a set upchannel.
 5. A cell according to claim 4 wherein said set up channeltransmitting means comprises: a set up transmitter for generating asignal; a power splitter coupled to said set up transmitter forsplitting said signal into a plurality of splitted signals; and meansfor directing said plurality of splitted signals to their correspondingzones.
 6. A cell according to claim 4 wherein said set up channelreceiving means comprises at least one combiner for combining signalsreceived by said antenna sets and a set up receiver for recoveringsignals transmitted by said mobile telephones.
 7. A cell according toclaim 1 wherein the number and position of the antenna sets are chosento allow propagation of signals in all areas of said cell so as to allowcomprehensive cell coverage at reduced overall power levels.
 8. A cellaccording to claim 1 further including reception coupling means forcoupling said signals from all the zones in the cell to said combiner.9. A cell according to claim 8 wherein said reception coupling meansfurther comprises time delay means for delaying among each other saidsignals from all the zones in the cell by a preselected amount so as toreduce interference caused by successive reception.
 10. A cell accordingto claim 1 wherein said receiver comprises a two-branch diversityreceiver.